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are intimately connected. If speciation is true, for instance, then common ancestry must also be true. But some parts are independent of others. Evolution might occur, for example, but it need not occur gradually. Some “mutationists” in the early twentieth century thought that a species could instantly produce a radically different species via a single monster mutation. The renowned zoologist Richard Goldschmidt, for example, once argued that the first creature recognizable as a bird might have hatched from an egg laid by an unambiguous reptile. Such claims can be tested. Mutationism predicts that new groups should arise instantly from old ones, without transitions in the fossil record. But the fossils tell us that this is not the way evolution works. Nevertheless, such tests show that different parts of Darwinism can be tested independently.
    Alternatively, evolution might be true, but natural selection might not be its cause. Many biologists, for instance, once thought that evolution occurred by a mystical and teleological force: organisms were said to have an “inner drive” that made species change in certain prescribed directions. This kind of drive was said to have propelled the evolution of the huge canine teeth of saber-toothed tigers, making the teeth get larger and larger, regardless of their usefulness, until the animal could not close its mouth and the species starved itself to extinction. We now know that there’s no evidence for teleological forces—saber-toothed tigers did not in fact starve to death, but lived happily with oversized canines for millions of years before they went extinct for other reasons. Yet the fact that evolution might have different causes was one reason why biologists accepted evolution many decades before accepting natural selection.
    So much for the claims of evolutionary theory. But here’s an important and commonly heard refrain: Evolution is only a theory, isn’t it? Addressing an evangelical group in Texas in 1980, presidential candidate Ronald Reagan characterized evolution this way: “Well, it is a theory. It is a scientific theory only, and it has in recent years been challenged in the world of science and is not yet believed in the scientific community to be as infallible as it once was believed.”
    The key word in this quote is “only.” Only a theory. The implication is that there is something not quite right about a theory—that it is a mere speculation, and very likely wrong. Indeed, the everyday connotation of “theory” is “guess,” as in, “My theory is that Fred is crazy about Sue.” But in science the word “theory” means something completely different, conveying far more assurance and rigor than the notion of a simple guess.
    According to the Oxford English Dictionary, a scientific theory is “a statement of what are held to be the general laws, principles, or causes of something known or observed.” Thus we can speak of the “theory of gravity” as the proposition that all objects with mass attract one another according to a strict relationship involving the distance between them. Or we talk of the “theory of relativity,” which makes specific claims about the speed of light and the curvature of space-time.
    There are two points I want to emphasize here. First, in science, a theory is much more than just a speculation about how things are: it is a well-thought-out group of propositions meant to explain facts about the real world. “Atomic theory” isn’t just the statement that “atoms exist”; it’s a statement about how atoms interact with one another, form compounds, and behave chemically. Similarly, the theory of evolution is more than just the statement that “evolution happened”: it is an extensively documented set of principles—I’ve described six major ones—that explain how and why evolution happens.
    This brings us to the second point. For a theory to be considered scientific, it must be testable and make